1. Field of the Invention
This invention relates to a clutch. More particularly the invention relates to a clutch having an inner part and an outer part which provide opposed races between which rollers (which may be in the form of needles) are received, means being provided to cause the rollers to jam on the races when torque is to be transmitted through the clutch.
2. Summary of the Invention
An object of the invention is to provide a simple clutch and one which may advantageously be combined with a viscous shear coupling for use in the driveline of a motor vehicle.
According to the invention we provide a clutch comprising concentric inner and outer parts rotatable about a common axis and having respectively outwardly and inwardly facing cylindrical races, a plurality of cylindrical rollers interposed between and engaging said races, a cage arranged to hold the rollers in circumferentially spaced relation, the cage comprising two separate parts, one part embracing the one ends of the rollers and the other part embracing the other ends of the rollers, resilient means for connecting said cage parts and biasing them to relative positions in which the longitudinal axes of the rollers are parallel to said common axis so that said rollers may roll around the races, and brake means associated with one of said parts and operable when the cage is rotating about said cage common axis to retard said one cage part relative to said other cage part so that the rotary axes of the rollers become inclined to said common axis and the rollers jam between the inner and outer races.
When the brake means is not operative, the rollers merely roll round the races and form, in effect, a roller bearing. In this state, the longitudinal axes of the rollers are parallel to the common axis of rotation between the parts of the clutch. When the brake is applied, the longitudinal axes of the rollers become inclined to the common axis and the rollers jam between the races, the rollers jamming at their centers on the inner race and at their ends on the outer race. In this position, torque can be transmitted between the two parts in either direction from the outer part to the inner part or the inner part to the outer part.
An advantage of the construction is that it is substantially wear-free. When the brake is not energized the rollers merely roll round the races and when the brake is energized there is no relative movement between the races and the rollers.
In a preferred arrangement, the brake means comprises a disc which is connected to one of said cage parts and which is subjected to a retarding force when the brake is operated thus to cause tilting of the roller axes. Preferably the braking means is a non-contact brake, i.e. one with no physical contact between the parts of the brake means. Thus in one embodiment the brake may be a hysteresis brake and include a magnet and means to move the magnet towards and away from the disc. The disc will be a material having a high magnetic hysteresis with a low coercive field strength and a comparatively high remanence and permeability. Full braking torque occurs at relatively low speeds and remains approximately constant with increase in speed.
The permanent magnet may be annular and mounted in a soft iron sleeve and arranged so that a shaft connected to the inner member passes rotatably through the central aperture of the magnet, the latter being non-rotatably mounted to be slidable relative to the shaft.
In another embodiment the brake is an eddy current brake and in this arrangement an electromagnet embraces the edge of the disc.
The resilient means between the cage parts may be in the form of leaf springs extending between the cage parts and circumferentially spaced around the cage.
The cage parts may be provided with recesses for receiving the ends of the rollers. Preferably the cage parts are made of a plastics material e.g. a polyamide.
The rollers may advantageously have a diameter to length ratio of between 1:1 and 1:20.
The invention also provides the combination of a clutch as described above with a viscous shear coupling comprising a housing, which is connected to the outer part of the clutch, and a hub rotatably mounted in the housing and connected to an output shaft, said coupling including a set of outer plates splined to the housing in spaced apart relation and a set of inner plates splined to the hub to be axially movable relative thereto, the plates of the two sets being interleaved and the housing being at least partially full of a viscous liquid, e.g. a silicone oil.
In this arrangement the hub may extend from the housing and may rotatably support, by means of a bearing, one end of a drive shaft which fixedly carries said inner part and said brake disc where provided, said outer part being connected to said housing by a hollow coupling which surrounds said bearing. The drive shaft may have a hollow end portion in which said bearing is received.
The invention also provides a drive transmission for a motor vehicle comprising a prime mover and a driveline between the prime mover and road wheels of the vehicle, the driveline including the combination of a clutch and a viscous shear coupling as described in which the inner part of the clutch is connected to the prime mover and the hub of the viscous shear coupling is connected to the road wheels. If the vehicle is a four-wheel drive vehicle the hub of the viscous coupling is connected to the set of wheels which is not permanently driven.
The combination of a clutch and a viscous shear coupling may also be combined with a differential gear and act across the differential gear to form a controllable, limited slip differential. The differential gear may be an inter-axle differential or an inter-wheel differential.